1. Field of the Invention
The invention relates to dual electromechanical servo actuators with model monitoring to provide fail operational characteristics.
2. Description of the Prior Art
Dual electromechanical servo actuators are known in the art and are exemplified by those disclosed in Applicant' assignee's pending U.S. patent application Ser. No. 811,653 entitled "Fail Passive Dual Servo With Continuous Motor Speed and Acceleration Monitoring" by M. T. DeWalt, filed June 30, 1977; Applicants' assignee's U.S. Pat. No. 4,035,705 entitled "Fail-Safe Dual Channel Automatic Pilot With Maneuver Limiting" By H. Miller issued July 12, 1977 and applicants' assignee's U.S. Pat. No. 3,504,248 entitled "Dual Channel Servo System Having Torque Equalization" by H. Miller issued Mar. 31, 1970.
These dual electromechanical servo actuators comprise two channels each including an electric servo motor driven by a servo amplifier. The motor output shafts are coupled through differential gearing to drive the output member. A tachometer generator, normally an integral part of each motor, provides an output shaft rate feedback signal to the input of the associated servo amplifier. A brake is included on the output shaft of each servo motor to clamp the shaft thereby disabling the channel.
These prior art electromechanical servo actuators exhibit fail passive or fail safe characteristics in that should one of the channels fail, various means are provided in the prior art arrangements for applying the brake of the defective channel permitting the operative channel to continue to position the output member through the differential gearing. In a commercial aviation environment, a second failure (such as a hardover) in the remaining channel could result in catastrophic loss of the aircraft. Thus, commercial aviation regulatory agencies require that after the first failure has occurred, the flight control system, of which the servo actuator is a part, be disengaged with manual control assumed by the pilot.
Although the above discussed dual electromechanical servo actuators adequately provide the intended performance as fail safe actuators, it is desirable under certain circumstances to provide fail operational performance with such actuators without an attendant increase in complexity, expense, bulk or weight.
As discussed in said Ser. No. 811,653, it is desirable to detect failures as rapidly as possible and quickly advise the pilot of the failed channel even though during the detection and advisory period the control surface will not significantly deflect due to the torque transmission characteristic of the differential. Although the servo actuator of said Ser. No. 811,653 adequately achieves this particular objective, the servo of Ser. No. 811,653 is fail passive rather than fail operative. Thus, not only is rapid failure detection desirable, but a high probability of detecting the failures and isolating the failures is also desirable, particularly for servo actuators for positioning aircraft primary control surfaces. Fast detection of failure and high reliability as therefore desiderata of the present invention.